This application claims the priority benefit of Taiwan application serial no. 91116088, filed Jul. 19, 2002.
1. Field of Invention
The present invention relates to the driving circuit of a display. More particularly, the present invention relates to the driving circuit of a display capable of preventing charge accumulation.
2. Description of Related Art
People are always interested in watching recorded images and movies. Ever since the invention of cathode ray tube (CRT), television has become commercialized and television sets are owned by almost every family. With rapid progress in technology, the CRT has been used for many applications including the desktop monitor of a personal computer. However, due to radiation hazards and the bulkiness of the electron gun, the CRT display is hard to make lighter and flatter.
Because of intrinsic bulkiness, researchers are now developing more slim-line displays. The so-called xe2x80x9cflat panel displaysxe2x80x9d now include liquid crystal displays (LCDs), field emission displays (FEDs), organic light-emitting diode (OLED) displays and plasma display panel (PDP) displays.
The organic light-emitting diode (OLED) is also known as an organic electroluminescence display (OELD) due to its self-illuminating character. OLED is driven by a low DC voltage and has properties including high brightness level, high energy efficiency, high contrast values as well as slim and lightweight. Moreover, the display is able to emit light of a range of colors from the three primary colors red (R), green (G) and blue (B) to white light. Hence, OLED is considered to be the display panel of the next generation. Aside from having high resolution and light just like the LCD and having self-illuminating capacity, a quick response and a low energy consumption just like the LED, OLED also has other advantages including a wide viewing angle, good color contrast and a low production cost. Thus, OLED is often used in LCD or as a background light source for indicator panels, mobile phones, digital cameras and personal digital assistants (PDA).
According to the type of driver selected to drive the OLED, the OLED can be divided into passive matrix driven or active matrix driven type. Passive matrix OLED has the advantage of structural simplicity and a low production cost. However, the passive matrix OLED has a relative low resolution rendering it unsuitable for producing high-quality images. Moreover, the passive matrix OLED consumes a lot of power, has a shorter working life and sub-optimal displaying capacity. On the other hand, although the active matrix OLED is slightly more expensive to produce, it can be assembled to form a huge screen, aside from having a large viewing angle, the capacity for producing high brightness level and a quick response.
According to the driving method, a flat display panel is divided into a voltage-driven type or a current-driven type. The pixel circuit of a conventional voltage-driven type of active matrix OLED is shown in FIG. 1. As shown in FIG. 1, the pixel circuit 10 includes a driving circuit 102 and an OLED (104). The driving circuit 102 further includes a thin film transistor TFT1 (106), a storage capacitor C (108) and a second thin film transistor TFT2 (110). The drain terminal of the transistor TFT1 (106) is coupled to a data line. The gate terminal of the transistor TFT1 (106) is coupled to a scanning line. The drain terminal of the transistor TFT1 (106) is coupled to a first terminal of the capacitor C (108) and the gate terminal of the transistor TFT2 (110). The drain terminal of the transistor TFT2 (110) is coupled to a voltage source V+, wherein the voltage V+ is a positive voltage. The source terminal of the transistor TFT2 (110) is coupled to the second terminal of the capacitor C (108) and the anode of the OLED (104) (also known as indium-tin-oxide, ITO). The cathode of the OLED (104) is coupled to another voltage source Vxe2x88x92. The voltage Vxe2x88x92 is a negative voltage or a ground potential. In FIG. 1, after forming the substrate of the transistor TFT1 (106) and the transistor TFT2 (110), an OLED (104) film is plated over the substrate. Hence, some electric charges are trapped on the anode of the OLED (104). If too many electric charges accumulate on the anode of the OLED of a particular pixel, that pixel no longer lights up leading to a point defect. In general, tens and sometimes hundreds of point defects are found within an area 50 cm2 of a display panel. When a large number of point defects appears on a display panel, quality of the image will be greatly compromised.
Accordingly, one object of the present invention is to provide the driving circuit of a display capable of preventing charge accumulation. Two thin film transistors are added to the driving circuit of each pixel of the display so that electric charges accumulated at the anode of a light-emitting diode during fabrication are dissipated and hence very few point defects are produced.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides the driving circuit of a display capable of preventing charge accumulation. The driving circuit drives a light-emitting device that has an anode and a cathode. The driving circuit includes a first transistor, a second transistor, a third transistor and a fourth transistor. The first transistor has a drain terminal, a gate terminal and a source terminal. The drain terminal of the first transistor is coupled to a data line. The gate terminal of the first transistor is coupled to a scanning line. The storage capacitor has a first terminal and a second terminal. The first terminal of the capacitor is coupled to the source terminal of the first transistor. The second terminal of the capacitor is coupled to the anode of the light-emitting device. The second transistor has a drain terminal, a gate terminal and a source terminal. The drain terminal of the second transistor is coupled to a first voltage source. The gate terminal of the second transistor is coupled to the source terminal of the first transistor and the first terminal of the capacitor. The source terminal of the second transistor is coupled to the anode of the light-emitting device and the second terminal of the capacitor. The third transistor has a drain terminal, a gate terminal and a source terminal. The drain terminal of the third transistor is coupled to the first voltage source and the drain terminal of the second transistor. The gate terminal of the third transistor is coupled to the source terminal of the third transistor, the anode of the light-emitting device and the second terminal of the transistor. The fourth transistor has a drain terminal, a gate terminal and a source terminal. The drain terminal of the fourth transistor is coupled to the gate terminal of the third transistor, the source terminal of the third transistor, the source terminal of the second transistor, the anode of the light-emitting device and the second terminal of the capacitor. The gate terminal of the fourth transistor is coupled to the source terminal of the fourth transistor, the cathode of the light-emitting diode and the second voltage source. During normal operation, the first voltage source is at a greater voltage than the anode and the second voltage source is at a smaller voltage than the anode.
In one embodiment of this invention, the third transistor is an N-type thin film transistor or a P-type thin film transistor.
In one embodiment of this invention, the fourth transistor is an N-type thin film transistor or a P-type thin film transistor.
In one embodiment of this invention, the display is an active matrix organic electroluminescence display.
In one embodiment of this invention, the first voltage and the second voltage are provided through a power supplier.
In one embodiment of this invention, the light-emitting device includes an organic light-emitting diode or a polymeric light-emitting diode.
The invention also provides a display capable of preventing charge accumulation. The display includes a plurality of pixels. Each pixel includes a first transistor, a storage capacitor, a second transistor, a third transistor, a fourth transistor and a light-emitting device. The first transistor has a drain terminal, a gate terminal and a source terminal. The drain terminal of the first transistor is coupled to a data line. The gate terminal of the first transistor is coupled to a scanning line. The storage capacitor has a first terminal and a second terminal. The first terminal of the capacitor is coupled to the source terminal of the first transistor. The second transistor has a drain terminal, a gate terminal and a source terminal. The drain terminal of the second transistor is coupled to a first voltage source. The gate terminal of the second transistor is coupled to the source terminal of the first transistor and the first terminal of the capacitor. The source terminal of the second transistor is coupled to the second terminal of the capacitor. The third transistor has a drain terminal, a gate terminal and a source terminal. The drain terminal of the third transistor is coupled to the first voltage source and the drain terminal of the second transistor. The gate terminal of the third transistor is coupled to the source terminal of the third transistor, the source terminal of the second transistor and the second terminal of the capacitor. The fourth transistor has a drain terminal, a gate terminal and a source terminal. The drain terminal of the fourth transistor is coupled to the gate terminal of the third transistor, the source terminal of the third transistor, the source terminal of the second transistor and the second terminal of the capacitor. The gate terminal of the fourth transistor is coupled to the source terminal of the fourth transistor and the second voltage source. The light-emitting device has an anode and a cathode. The anode is coupled to the second terminal of the capacitor, the source terminal of the second transistor, the source terminal of the third transistor and the gate terminal of the third transistor. The cathode is coupled to the second voltage source, the source terminal of the fourth transistor and the gate terminal of the fourth transistor. During normal operation, the first voltage source is at a greater voltage than the anode and the second voltage source is at a smaller voltage than the anode.
In brief, this invention provides two additional thin film transistors to the driving circuit of each pixel of the display. When positive electric charges accumulate at the anode of a light-emitting diode during fabrication so that the anode has a potential larger than the common positive voltage line of the panel, a current will flow from one of the thin film transistors to the positive voltage line. Conversely, when negative electric charges accumulate at the anode so that the anode has a potential smaller than the common negative voltage line of the panel, a current will flow from the common negative voltage line to the anode via the other thin film transistor to neutralize the negative charges. Therefore, very few electric charges will accumulate at the anode of the light-emitting device and hence very little point defects will appear on the display panel.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.